SF: Confine display serial number to EDID parser

The EDID parser falls back to the serial number if the display name is
missing. Previously, this string was only used to compute a hash for
display ID encoding, but is now propagated through DeviceProductInfo.

Though DeviceProductInfo is currently a hidden API, stop exposing the
serial number in the first place to avoid inadvertently leaking it.

Bug: 138677816
Test: libsurfaceflinger_unittest
Change-Id: I13f4e7f4d0e9e90866b74ac33a048e8809158a43